The present invention relates to an injection-molded article which has a hollow portion and satisfies the requirement for a high-quality appearance.
Exterior parts of an automobile and housings and covers of business machines and electric household appliances (to be generically referred to as a "product" hereinafter) are required to have high-quality appearances. These products are generally produced by injection molding, and they have base bodies defining their shape which have flat and/or curved surfaces, bent portions, corner portions and so on. Further, these products sometimes have rib-shaped thick-wall portions which are integrally formed with their base bodies to strengthen the base bodies as required. In this case, however, a so-called sink mark generally appears on that surface of the base body which is opposite to the thick-wall portion (to be sometimes simply referred to as a "product surface" hereinafter), and the resultant product has not an excellent appearance.
For preventing the occurrence of the sink mark, there is known a so-called gas injection molding method in which a pressurized gas is introduced into a molten resin after or while the molten resin is injected into a cavity of a mold, as is disclosed in JP-A-63-268611 (corresponding to U.S. Pat. No. 4,923,666) for example. In this gas injection molding method, a molded article has a rib-shaped thick-wall portion or a thick-wall corner portion, and a hollow portion is formed in the thick-wall portion or the thick-wall corner portion.
When the product is produced according to the above gas injection molding method, for example, the hollow portion is formed in the thick-wall portion having a larger width and a larger height than a thickness of a base body. FIGS. 17A, 17B and 17C show schematic cross sections of such products. In each of FIGS. 17A, 17B and 17C, numeral 10 indicates a base body, 20 indicates a thick-wall portion, and 30 indicates a hollow portion. The hollow portion 30 is formed generally at the foot of the thick-wall portion 20, and it sometimes extends into the base body 10.
The hollow portion can be formed by the following steps. After or when a molten resin is injected into a cavity of a mold, a pressurized gas is introduced into that part of the molten resin which corresponds to the hollow portion of the thick-wall portion. Then, the pressure of the pressurized gas is maintained until the molten resin is solidified and cooled in the cavity, whereby the hollow portion is formed in the thick-wall portion. Further, since the pressurized gas keeps on pressing the resin to the inner wall of the cavity while the molten resin is solidified and cooled, it is possible to prevent the occurrence of the sink mark on that surface 12 (product surface) of the base body which is opposite to a portion where the thick-wall portion is formed.
When the product is produced by the gas injection molding method as described above, the sink mark can be prevented from being formed on the surface 12 (product surface) of the product opposite to the thick-wall portion, and the base body can be imparted with high strength due to the thick-wall portion. Moreover, the overall weight of the product can be decreased due to the hollow portion formed in the thick-wall portion. Therefore, the gas injection molding method is often employed for producing various parts of an automobile and parts of business machines and an electric household appliance.
A technique for forming an injection-molded article by the gas injection molding method to prevent the occurrence of a sink mark on a product surface opposite to a portion where a rib is formed is also disclosed in "Gasinnendruck zum Ausblasen uberschussiger Schmelze", Kunststoffe 80 (1990) 8, pp. 873-876, "Neue Moglichkeiten beim Spritzgie.beta.en durch das Gasinnendruckverfahren", Kunststoffe 79 (1989) 11, pp. 1102-1107, or "Gas injection finally freed for growth", EUROPEAN PLASTICS NEWS, May 1991, pp. 61-85.
Further, "Design guidelines for gas injection molding", PLASTIC DESIGN FORUM, July/August 1990, pp. 41, describes a thick-wall portion of a molded article formed by a gas injection molding method. This molding article has a rib projecting from a base body and a thick-wall portion formed between the rib and the base body. This article discloses many design examples of the shapes of the thick-wall portions.
However, when any one of the above gas injection molding methods is employed for molding the product, a strain on the base body is sometimes observed. The strain occurs on a surface opposite to an interface between the thick-wall portion and the base body where the thick-wall portion is formed (to be sometimes referred to as "product surface portion" hereinafter), and this strain in the form of a continuous linear, very small groove is observed on the product surface portion along the circumference of a foot portion of the thick-wall portion. This strain can be identified by observing a distortion of an image reflected on the surface of the product. Such a strain does not specially matter when a product is used as an interior fitting part of an automobile and a functional part or a general cover of a business machine and electric household appliances.
However, when it comes to a variety of exterior parts of an automobile and a housing and a special cover of a business machine and an electric household appliance which are required to have high surface smoothness or an excellent appearance, for example, a strain having a depth of about 3 .mu.m and a width of about 5 mm causes a defective appearance if it occurs on the product surface portion. In particular, even a slight strain can be observed, if the product is a molded article formed from a resin having gloss or a molded article coated with a coating composition having gloss.
A molded article produced by any one of the above gas injection molding methods sometimes shows a slightly irregular color on the product surface portion. When a molten resin is injected and when a pressurized gas is introduced, the molten resin flows from a mold cavity portion corresponding to the thick-wall portion to a mold cavity portion corresponding to the base body. In this case, the flowing state of the molten resin greatly changes, and the flow of the molten resin relative to a cavity wall surface is disordered. As a result, a molded article shows some haze, which is called irregular color.
The above irregular color (non-uniformity in color) is observed in the form of a continuous line or a spread on the product surface portion along the circumference of a foot portion of the thick-wall portion. In exterior fitting parts of an automobile or housings or covers of business machines or an electric household appliance which are required to have a high-quality appearance, the above irregular color results in a defective appearance.
In view of a strain or an irregular color, it is very difficult to apply conventional gas injection molding methods used to remove a so-called sink mark to the production of a product required to have a special high-quality appearance.